What possible reason would the UN have to do this? Actually, what possible reason would a government have to do this as well?
Governments are not in the business of being unpopular - this is why governments slowly become more and more inclusive. You can see this for example in the moves by governments to legalize gay marriage and pot smoking. The only governments who can get away with making religion illegal are dictatorships because they know the people have no choice in the matter. So before religion becomes illegal in the US for example, we'll have to see the country become a dictatorship. I'll wait for that to happen first before i worry about religion.